High-mobility group box 1 protein, i.e., HMGB1 protein (hereinafter referred to as “HMGB1”), is a protein essential for survival (Non-Patent Literature 1), and HMGB1 functions in a cell nucleus as a protein that forms a chromatin structure required for initiation of transcription (Non-Patent Literature 2). When HMGB1 is leaked or secreted out of a cell, HMGB1 functions as a cytokine that facilitates an inflammatory reaction, a morphological change of a cell, and migration of a cell (Non-Patent Literature 3). In recent years, an analysis of extracellular functions of HMGB1 has advanced, and it has been revealed that when autoimmune diseases, sepsis, traumatic shock, ischemia, and ischemia reperfusion disorders occur, HMGB1 aggravates inflammatory reactions or induces apoptosis. Accordingly; it has been demonstrated that HMGB1 serves as a target substance and a diagnostic marker of the treatment of these diseases (Non-Patent Literature 4). Moreover, regarding cancers, it has been reported that HMGB1 is involved in proliferation and metastasis infiltration of breast cancer, colon cancer, melanoma, prostatic cancer, pancreatic cancer, lung cancer, and the like, and the level of HMGB1 expression is increased compared with that in normal tissue (Non-Patent Literature 5). Under this circumstance, it has been desired that a substance that can bind to HMGB1 and neutralize the function thereof in order to prevent and treat the aforementioned diseases. However, only a diagnostic method that uses a monoclonal anti-HMGB1 antibody and a cerebral infarction model treatment in laboratory animals have been shown to be effective so far (Patent Literature 1 and 2, Non-Patent Literature 6).